The present invention relates to air-fuel ratio control apparatus for internal combustion engines, and more particularly to apparatus for optimally controlling the air-fuel ratio of an engine in accordance with the engine operating conditions so as to prevent thermal breakdown of the supercharger and the exhaust system components and reduce fuel consumption.
Generally, the compression of an intake mixture supplied to an engine by a supercharger is accompanied with an increase in the intake air temperature supercharging necessitates decreasing the turbine inlet nozzle area for exhaust gases as far as possible so as to ensure rise in the supercharged pressure and hence the supercharging effect at the low engine speeds. However this increases exhaust pressure at high engine speeds and increases the tendency of knocking with ordinarily used gasoline fuel.
More specifically, under high speed and high load operating conditions the engine is operated with an ignition timing greatly retarded with respect to the optimum ignition timing, MBT (minimum spark advance for best torque) for the air-fuel mixture supplied to the combustion chamber. As a result, the exhaust gas temperature becomes extremely high so that the supercharger installed in the exhaust system and the exhaust system components are exposed to elevated temperatures and are faced with the danger of breaking down by heat. At the same time, output power is restricted by knocking with a resulting increase in fuel consumption. Thus, to overcome these deficiencies, it is already known in the art to use either fuels having a large ignition lag, in place of ordinary gasoline, so as to increase the ignition lag required for initial combustion or an additive such as tetraethyl lead or a mixture of a fuel having a good antiknock properties (e.g., benzol or alcohols) and ordinary gasoline. However, these measures are not practical in that a change of fuel is not desirable so far as the matter concerns gasoline engines and the addition of lead is contrary to the recent trend toward using clear gasolines. Also the addition of any other fuel to ordinary gasoline requires a device for this particular purpose with the resulting increase in the cost. Further, recently the use of clear gasolines must be a prerequisite in view of the demand for minimized emissions.
Other measures intended for pushing back the ignition timing at which knock begins to occur have been investigated and they include reducing the compression ratio to minimize the temperature and pressure of the exhaust gas in the combustion process, using a variable compression ratio construction, modifying the combustion chamber into a swirl producing construction which increases the flame speed, etc. However, the reduced compression ratio decreases fuel consumption under part-load operation, and the variable compression ratio construction and the modified combustion chamber shape are disadvantageous from the standpoints of structural complexity, reliability and increased cost.
Where the use of a clean gasoline is presupposed and the compression ratio is not modified particularly, the most simple and inexpensive method of decreasing the elevated exhaust gas temperature under high speed and high load operation is to vary the air-fuel ratio of mixture. In the case of gasoline engines, generally the air-fuel ratio at high engine speed and load, is kept considerably rich as compared with that at low engine speed and load so as to push back the knock limit to thereby decrease the exhaust gas temperature and restore the output power. In this case, depending on the air-fuel ratio control method used, there is the danger that a supercharger installed in the exhaust system and the exhaust system components may be damaged by heat, with the fuel consumption of the engine being inevitably increased.